1. Field of the Invention
The present invention relates to a magnetic head to record and reproduce information on and from a magnetic recording medium, and more particularly to an improved giant magnetoresistive sensor and a magnetic recording/reproducing apparatus equipped with said sensor.
2. Description of the Related Art
The increasing magnetic recording density requires a highly sensitive magnetic head for reproduction. The one meeting this requirement is described in xe2x80x9cGiant magnetoresistance in soft magnetic multi-layer filmxe2x80x9d, Physical Review B, vol. 43, pp. 1297-1300. It is constructed such that two magnetic layers are separated by one non-magnetic layer and an exchange bias magnetic field is applied to one of the magnetic layers from an antiferromagnetic layer. This type of multi-layer film has resistance R with a component varying in proportion to cosxcex8, with xcex8 being an angle between the directions of magnetization of the two magnetic layers, according to the aforesaid thesis. This phenomenon is referred to as giant magnetoresistance (GMR).
A conventional giant magnetoresistive sensor is shown in FIG. 7. It consists of a substrate 5 and several layers sequentially formed thereon. Adjacent to the substrate are a magnetic shield layer 10 and a magnetic gap layer 20. On the magnetic gap layer 20 is a magnetoresistive film 30, which consists of a ferromagnetic film (free layer) 35, a copper layer 40, a ferromagnetic film (pinned layer) 65, and an antiferromagnetic film 70, which are formed sequentially one over another. The arrow 55 indicates the direction of magnetization. With the magnetoresistive film 30 patterned, there are arranged an electrode film 90 and a permanent magnet layer 80 at each side thereof. The top is covered with a magnetic gap layer 100 and a magnetic shield layer 110. The magnetoresistive film mentioned above is characterized in that the pinned layer has its magnetization pinned in the direction of element height (depth) by the exchange bias magnetic field from the antiferromagnetic layer. In general, the free layer has the axis of easy magnetization parallel to the cross-track direction (z direction) of the head.
In the case of the head mentioned above, it is desirable that the magnetization in the entire free layer be kept parallel to the cross-track direction of the head so that the free layer does not suffer magnetic saturation when a signal magnetic field from the medium is applied upward and downward in the direction of the element height of the head. Unfortunately, the magnetization in the free layer does not become uniformly parallel to the cross-track direction of the head because the free layer receives a static magnetic field which occurs as the pinned layer (orienting vertically to the medium surface) becomes magnetized. The result is that the head becomes sensitive unequally to the positive and negative magnetic fields and reproduces a large peak asymmetry of read-back waveform. This not only adversely affects the improvement of error rate by signal processing such as PRML (partial response sampling plus maximum likelihood detection) but also lowers the output. The peak asymmetry of read-back waveform is defined as follows.
Asym.=|V+xe2x88x92Vxe2x88x92|/|V++Vxe2x88x92|
(where V+ denotes the peak value of the positive output and Vxe2x88x92 denotes the peak value of the negative output.)
There is disclosed in Japanese Patent Laid-open No. 169026/1995 a giant magnetoresistive sensor designed to reduce the peak asymmetry of read-back waveform. As shown in FIG. 8, it has a magnetoresistive film 30 consisting of a ferromagnetic film (free layer) 35, a copper layer 40, a composite ferromagnetic film (pinned layer) 50, and an antiferromagnetic film 70. The pinned layer 50, which is a composite ferromagnetic film, consists of two ferromagnetic films 51 and 53 (of Co or the like) and a non-magnetic layer 52 (or Ru or the like), the former having their magnetization strongly coupled in the antiparallel direction through the latter. The two ferromagnetic films produce magnetic moments aligning in the antiparallel direction, thereby canceling out each other. The result is a reduction of static magnetic field applied to the free layer from the pinned layer. The second ferromagnetic film 53 of the pinned layer 50 has its magnetization pinned by the antiferromagnetic film 70.
There is disclosed in Japanese Patent Laid-open No. 7235/1996 another giant magnetoresistive sensor designed to reduce the peak asymmetry of read-back waveform. As shown in FIG. 9, it has a magnetoresistive film 30 consisting of a ferromagnetic film (free layer) 35, a copper layer 40, and a composite ferromagnetic film (pinned layer) 50. The pinned layer 50, which is a composite ferromagnetic film, consists of two ferromagnetic films 51 and 53 (of Co or the like) and a non-magnetic layer 52 (or Ru or the like), the former having their magnetization strongly coupled in the antiparallel direction through the latter, like the aforesaid head. The two ferromagnetic films 51 and 53 should have an adequate thickness, so that the pinned layer has a large effective coercive force for it to be of self-pinned type. The result is a reduction of static magnetic field applied from the pinned layer and obviation of the antiferromagnetic film to fix the pinned layer. The advantage is a reduction of the entire film thickness of the head and a reduction of the gap length.
On the other hand, there is disclosed in Japanese Patent Laid-open No. 347013/1993 and U.S. Pat. No. 5,287,238 a giant magnetoresistive sensor designed to increase its reproducing output. As shown in FIG. 10, it has a magnetoresistive film 30 consisting of a first antiferromagnetic layer 70, a first pinned ferromagnetic film 65, a non-magnetic film 40, a free ferromagnetic film 35, a non-magnetic layer 40, a second pinned ferromagnetic film 66, and a second antiferromagnetic film 71. The multi-layer structure, with the free layer being held between the pinned layers, causes electrons to scatter over a larger area of interface. This tends to a larger relative change of magnetoresistance (xcex94R/R in percent) and a larger output of reproduction. This type of giant magnetoresistive sensor is called dual spin valve (SV) head.
Another type of dual spin valve (SV) head is disclosed in Japanese Patent Laid-open No. 225925/1995. As shown in FIG. 11, it has a magnetoresistive film 30 consisting of a first free magnetic film 35, a non-magnetic film 40, a first ferromagnetic pinned film 65, an antiferromagnetic film 70, a second ferromagnetic pinned layer 66, a non-magnetic film 40, and a second free magnetic film 36. As in the foregoing head, the multi-layer structure, with the antiferromagnetic film being held between the pinned layers and the free layers, causes electrons to scatter over a larger area of interface. This tends to a larger relative change of magnetoresistance (xcex94R/R in percent).
The disadvantage of the aforesaid structure, with the free layer or antiferromagnetic film being held between two upper and lower pinned layers, is that the static magnetic field applied to the thickness of the free layer from the pinned layer increases as the pinned layer increases. Consequently, the direction of magnetization of the free layer deviates from the direction of the track width of the head, with the result that the peak asymmetry of reproduced signals becomes larger. The larger the asymmetry, the lower the read-back output.
A dual spin valve film to remedy the peak asymmetry of read-back waveform is described in xe2x80x9cPtMn dual spin valve film with a Co/Ru/Co laminated ferri pinned magnetic layerxe2x80x9d, Synopsis of the 22nd Lecture Meeting of Japan Institute of Applied Magnetism, p. 309. As shown in FIG. 12, it has a magnetoresistive film 30 consisting of a first antiferromagnetic film 70, a first composite ferromagnetic film (pinned layer) 50, a non-magnetic film 40, a ferromagnetic film (free layer) 35, a non-magnetic film 40, a second composite ferromagnetic film (pinned layer) 60, and a second antiferromagnetic film 71. The composite ferromagnetic films 50 and 60 have the same structure as the aforesaid composite film shown in FIG. 8. This structure is intended to reduce the static magnetic field from the pinned layer, thereby remedying the peak asymmetry of the read-back waveform of the head.
The disadvantage of the composite film functioning as the upper and lower pinned layers in the dual spin valve head is that the overall film thickness of the magnetic head increases. Any attempt to compensate the increased thickness by reduction in the thickness of the magnetic gap layers 20 and 100 shown in FIG. 7 ends up with an insufficient electro-static durability which leads to electro-static destruction due to short-circuits between the magnetoresistive film and shield film.
It is an object of the present invention to provide a giant magnetoresistive sensor of dual spin valve type which excels in electro-static durability and peak symmetry of read-back waveform.
The giant magnetoresistive sensor has a magnetoresistive film consisting of a substrate, a first free ferromagnetic film, a first non-magnetic film, a composite ferromagnetic film, a second non-magnetic film, and a second free ferromagnetic film, which are formed sequentially one over another, so that the composite ferromagnetic film becomes the pinned layer of self-pinned type. The pinned layer of self-pinned type consists of a first, second, and third ferromagnetic films antiferromagnetically coupled with one another and films separating these three ferromagnetic films and antiferromagnetically coupling them with one another.
According to the present invention, the giant magnetoresistive sensor may also have a magnetoresistive film consisting of a substrate, a first composite ferromagnetic film, a first non-magnetic film, a free ferromagnetic film, a second non-magnetic film, and a second composite ferromagnetic film, which are laminated one over another. The aforesaid first and second composite ferromagnetic films should be the pinned layer of self-pinned type. The pinned layer of self-pinned type consists of a first and second ferromagnetic films antiferromagnetically coupled with each other and a film separating these two ferromagnetic films and antiferromagnetically coupling them with each other. Either of the first and second composite ferromagnetic films may be replaced by the conventional pinned layer in which a singe pinned ferromagnetic film is pinned by an antiferromagnetic film.
The giant magnetoresistive sensor of the present invention is characterized in that the net amount of magnetic moment of the aforesaid composite ferromagnetic film can be made smaller than the total amount of magnetic moment of each ferromagnetic film in the composite ferromagnetic film. The composite film is regarded as one magnetic entity responsible for the net magnetic moment.
Also, the giant magnetoresistive sensor of the present invention may be constructed such that the first and second ferromagnetic films of the aforesaid magnetic composite layer have almost the same magnetic moment and consequently the net magnetic moment of the aforesaid composite ferromagnetic film is nearly null.
The giant magnetoresistive sensor of the present invention should have the aforesaid antiferromagnetic film such that it produces great unidirectional anisotropy regardless of the order of lamination of the antiferromagnetic film and the ferromagnetic film. It should preferably be made of nickel oxide, PtMn, PtPdMn, CrMnPt, or the like.
In addition, the giant magnetoresistive sensor of the present invention may be combined with a thin-film head of induction type for magnetic recording so as to constitute a thin-film magnetic head.
The giant magnetoresistive sensor of dual spin valve head type may have a composite film of self-pinned type as the pinned layer. The result is a reduction of static magnetic field from the pinned layer and hence a remedy for peak asymmetry of read-back waveform. Another result is a reduction of the total film thickness of the magnetoresistive film. This permits the insulating film between the magnetoresistive film and shield film to be thicker, and the thicker insulating film contributes to electro-static durability.
While the conventional dual spin valve head shown in FIG. 11 has the disadvantage that the scattering of electrons not contributing to the relative change of magnetoresistance takes place in the antiferromagnetic film 70, the giant magnetoresistive sensor of the present invention shown in FIG. 1 is free from this disadvantage owing to the two free layers formed on both sides of the self-pinned layer and hence it has a better relative change of magnetoresistance than the conventional one.
In addition, according to the present invention, it is possible to control the peak symmetry of read-back waveform if the static magnetic field applied to the free layer from the pinned layer is regulated by changing the difference in thickness of the two or three ferromagnetic films in the composite film of the pinned layer of self-pinned type.